Cygwin dll extension and installation 

* Cygwin wants compiled Python modules to be X.dll.
* CMake puts dlls into the RUNTIME_OUTPUT_DIRECTORY.
* Since we are supporting cygwin as cygwin, explicitly disable CMake legacy W32 mode to avoid warnings.

This is 1, 3 and 4 in https://github.com/Valloric/YouCompleteMe/pull/866